The Key lime or acid lime is a citrus hybrid native to tropical Southeast Asia. It has a spherical fruit, 25–50 mm in diameter. The Key lime is usually picked while it is still green, but it becomes yellow when ripe.
The Key lime is smaller, seedier, has higher acidity, stronger aroma, and thinner rind than the Persian lime (Citrus × latifolia). It is valued for its characteristic flavor. The name comes from its association with the Florida Keys, where it is best known as the flavoring ingredient in Key lime pie. It is also known as West Indian lime, bartender's lime, Omani lime, or Mexican lime, the last classified as a distinct race with a thicker skin and darker green colour. Philippine varieties have various names, including dayap and bilolo.